This invention relates generally to steam turbines and more generally to methods and apparatus for low flow bucket tip cooling and moisture removal.
In known steam turbines low flow conditions (low VAN) sometime occur at startup and during high back pressure operation. The flow structure in the last stage of a steam turbine (L-0 stage) changes significantly during low VAN operation. This change is due to centrifugal forces acting on the buckets which can send steam upward, and can create tip and root recirculation zones along with the main steam flow. In the bucket root zone the flow is in a backwards direction, bringing cold and wet steam from the condenser into the steam path. In the tip (outer flowpath) steam recirculation provides significant heating impact on the bucket tip section due to “windage”. During low flow, high speed, operation, the flow near the bucket tip can become trapped and subsequently the steam is heated due to the bucket tip doing work on the steam that is trapped. This “windage” heating primarily takes place under startup low VAN conditions.
When low VAN operation is the result of high back pressure, the flow in this tip zone can also be subject to flow instability (unsteadiness) and pressure pulsation, which result in the L-0 bucket dynamic stresses increasing. At steady state operation, moisture can accumulate at the L-0 nozzle outer sidewall. Removal of this moisture can reduce last stage bucket (LSB) erosion.
The low VAN conditions described above can have a detrimental effect on the last stage bucket. The heating of the bucket tip area can reduce bucket life and reliability. It can also reduce the ability to use a hybrid bucket construction (polymer filler in the outer bucket area). Also, the instability from the low VAN conditions can cause pressure pulsations that could affect the bucket reliability. Additionally, excess moisture in the last stages sometimes accumulates on the outer sidewall, among other locations, of the last stage nozzle which can cause erosion of the nozzle.